Arrhythmia/Electrophysiology

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1 Arrhythmia/Electrophysiology Cardiac Electrophysiological Substrate Underlying the ECG Phenotype and Electrogram Abnormalities in Brugada Syndrome Patients Junjie Zhang, BS; Frédéric Sacher, MD; Kurt Hoffmayer, MD; Thomas O Hara, PhD; Maria Strom, PhD; Phillip Cuculich, MD; Jennifer Silva, MD; Daniel Cooper, MD; Mitchell Faddis, MD; Mélèze Hocini, MD; Michel Haïssaguerre, MD; Melvin Scheinman, MD; Yoram Rudy, PhD Background Brugada syndrome (BrS) is a highly arrhythmogenic cardiac disorder, associated with an increased incidence of sudden death. Its arrhythmogenic substrate in the intact human heart remains ill-defined. Methods and Results Using noninvasive ECG imaging, we studied 25 BrS patients to characterize the electrophysiological substrate and 6 patients with right bundle-branch block for comparison. Seven healthy subjects provided control data. Abnormal substrate was observed exclusively in the right ventricular outflow tract with the following properties (in comparison with healthy controls; P<0.005): (1) ST-segment elevation and inverted T wave of unipolar electrograms (2.21±0.67 versus 0 mv); (2) delayed right ventricular outflow tract activation (82±18 versus 37±11 ms); (3) lowamplitude (0.47±0.16 versus 3.74±1.60 mv) and fractionated electrograms, suggesting slow discontinuous conduction; (4) prolonged recovery time (381±30 versus 311±34 ms) and activation-recovery intervals (318±32 versus 241±27 ms), indicating delayed repolarization; (5) steep repolarization gradients (Δrecovery time/δx=96±28 versus 7±6 ms/cm, Δactivation-recovery interval/δx=105±24 versus 7±5 ms/cm) at right ventricular outflow tract borders. With increased heart rate in 6 BrS patients, reduced ST-segment elevation and increased fractionation were observed. Unlike BrS, right bundle-branch block had delayed activation in the entire right ventricle, without ST-segment elevation, fractionation, or repolarization abnormalities on electrograms. Conclusions The results indicate that both slow discontinuous conduction and steep dispersion of repolarization are present in the right ventricular outflow tract of BrS patients. ECG imaging could differentiate between BrS and right bundle-branch block. (Circulation. 2015;131: DOI: /CIRCULATIONAHA ) Key Words: Brugada syndrome electrocardiography electrophysiology Brugada syndrome (BrS) is an inherited disorder, affecting predominantly men in their forties 1 and associated with an increased incidence of sudden cardiac death. It presents with an ECG expression of atypical right bundle-branch block (RBBB) pattern and ST-segment elevation (STE) in leads V1 through V3. 1,2 It is estimated that BrS causes 20% of sudden cardiac death in cardiac patients with structurally normal hearts. 1 Understanding its pathophysiological mechanisms is essential for improving risk stratification, diagnosis, and treatment to prevent sudden cardiac death. Clinical Perspective on p 1959 BrS is considered a primary electric cardiac disease, because no structural anomalies are detected by conventional imaging. Up to 30% of patients test positive for mutations in the SCN5A gene, 1 which causes a loss-of-function of the cardiac sodium channel (I Na ). A Brugada ECG pattern can be provoked, in some affected patients with a normal baseline ECG pattern, by administration of I Na blockers. There are 2 leading hypotheses for mechanisms underlying BrS phenotype and arrhythmias. (1) The abnormal repolarization hypothesis (based on the canine wedge preparation) 3 5 : Phase 1 notch is present in epicardial action potentials (APs) owing to a high density of transient outward current (I to ), but absent in endocardial APs (low I to density). A reduced I Na in BrS exaggerates the phase 1 notch preferentially in right ventricular outflow tract (RVOT) epicardium, where I to is expressed with maximal density. The resulting voltage gradients give rise to Received October 16, 2014; accepted March 18, From Cardiac Bioelectricity and Arrhythmia Center (J.Z., P.C., J.S., D.C., M.F., Y.R.) and Department of Biomedical Engineering (J.Z., Y.R.), Washington University, St. Louis, MO; Bordeaux University Hospital, LIRYC Institute, Pessac, France (F.S., M. Hocini, M. Haïssaguerre); School of Medicine, University of Wisconsin, Madison (K.H.); Institute for Computational Medicine, Johns Hopkins University, Baltimore, MD (T.O.); CardioInsight Technologies, Cleveland, OH (M. Strom); School of Medicine, Washington University, St. Louis, MO (P.C., J.S., D.C., M.F., Y.R.); and School of Medicine, University of California, San Francisco (M. Scheinman). The online-only Data Supplement is available with this article at /-/DC1. Correspondence to Yoram Rudy, PhD, Director, Cardiac Bioelectricity and Arrhythmia Center, Campus Box 1097, One Brookings Dr, Washington University in St. Louis, St. Louis, MO rudy@wustl.edu 2015 American Heart Association, Inc. Circulation is available at DOI: /CIRCULATIONAHA

2 Zhang et al Cardiac EP Substrate in Brugada Syndrome Patients 1951 STE on the ECG. Further outward shift of the balance between I Na and I to can repolarize the membrane during phase 1 below the voltage range for L-type calcium channels (I Ca-L ) activation. When I Ca-L fails to activate, the AP loses its plateau (dome). Spatially heterogeneous loss of the AP plateau in the RVOT can lead to reentry (termed phase-2 reentry by Antzelevitch and colleagues). (2) The abnormal conduction hypothesis (based on whole-heart studies in BrS patients) 6 8 : Impaired I Na in structurally deranged tissue causes slow discontinuous AP propagation. Asynchronous activation can promote reentrant arrhythmias and create voltage gradients, causing STE and fractionation on the ECG. Data from catheter mapping 6 showed that delayed conduction and abnormal electrograms (EGMs) with low voltage and fractionated late potentials (reflecting slow discontinuous conduction) were exclusively localized in the anterior aspect of the RVOT epicardium. Catheter ablation in this area normalized the Brugada ECG and prevented ventricular tachycardia. The pathophysiological mechanisms of the abnormal ECG and arrhythmia in BrS are still a subject of debate. Although much is known at the molecular and cellular scales, understanding the cause of the BrS ECG pattern and associated arrhythmias requires detailed characterization of the electrophysiological (EP) substrate in the intact hearts of BrS patients. This requires high-resolution, panoramic EP mapping of the ventricles and cannot be achieved with invasive catheter mapping. The recent development of noninvasive mapping with electrocardiographic imaging (ECGI) allowed us to obtain the first high-resolution panoramic EP data from BrS patients, including up to 1500 unipolar epicardial EGMs and epicardial maps of activation and repolarization Based on these data recorded during sinus rhythm (SR), we characterized the EP substrate in BrS patients in an effort to provide insight into the mechanistic origin of the BrS phenotype. The results show that both repolarization and structurally based conduction abnormalities coexist in the hearts of BrS patients. We also compared the BrS EP substrate with non-brs RBBB (generally considered benign) to determine whether the substrate is specific to BrS, and whether ECGI can differentiate between these 2 pathologies with similar ECGs. Methods Patient Population Twenty-five BrS patients from 2 centers in the United States and 1 center in France were enrolled. Details on patient recruitment are provided in online-only Data Supplement Methods. The BrS diagnosis was based on the consensus criteria 1 and was the basis for patient recruitment. Beyond the consensus criteria, pregnant women and children were excluded. Patient demographic data are provided in onlineonly Data Supplement Table I. Six RBBB patients with a prolonged QRS >120 ms were studied for comparison. All BrS and RBBB patients had no evidence of structural heart disease on echocardiography or MRI. Data from 7 healthy subjects 12 provided normal control. Subject characteristics are provided in online-only Data Supplement Table II. Protocols were approved by the institutional review boards at the 3 centers; written informed consent was obtained from all patients. Noninvasive Mapping ECGI methodology was described previously 9 14 (Figure IA in the online-only Data Supplement). In brief, torso surface ECG potentials, recorded simultaneously from 250 electrodes, were combined mathematically with patient-specific heart-torso geometry from ECG-gated computed tomography to construct epicardial potentials, unipolar EGMs, and maps of epicardial activation and repolarization. Bipolar EGMs were constructed for fractionation analysis. The method was validated extensively for reconstruction of EGMs, 9,10 activation, 11,12 and repolarization. 10,12 Additional validation references are provided in online-only Data Supplement References. Spatial properties of the EP substrate were determined by dividing the epicardium into 6 segments based on computed tomography images (online-only Data Supplement Table III; the RVOT is depicted in Figure IB in the online-only Data Supplement). EGMs from valvular regions were excluded. On average, 1154 EGMs/patient were used for analysis. EGMs were evaluated for morphology, magnitude, and fractionation. EGM Brugada morphology was defined as STE followed by T-wave inversion. EGM magnitude was measured peak-to-peak during the EGM QRS; for fractionated EGMs, the measurement was confined to the fractionated segment. Fractionation was expressed as number of low-amplitude deflections per EGM and displayed on epicardial EGM deflection maps. 13 For the fractionation analysis, bipolar EGMs were approximated by time derivatives of unipolar EGMs. Local activation time (AT, referenced to beginning of QRS in ECG lead II) was determined by the maximal negative slope of the EGM during QRS inscription (Figure IC in the online-only Data Supplement). From the ATs, epicardial activation isochrone maps were created. Slow conduction is represented by crowded isochrones. Regional activation duration (AD) was defined as the interval between the earliest and latest AT in a region, considering all EGMs in that region. Local recovery time (RT) was determined from the maximal positive slope of the EGM T wave (Figure IC in the online-only Data Supplement); it reflects the sum of local activation time and local action potential duration (APD). For a given activation sequence, RT determines spatial voltage gradients during repolarization, and underlies ST-T deflections. RT dispersion provides substrate for unidirectional block and reentry. Activation-recovery interval (ARI) was defined as the difference between RT and AT. ARI is independent of AT and a surrogate for local APD. 16 Epicardial RT and ARI gradients were computed as the difference between neighboring epicardial nodes, divided by the distance between them. ECGI was conducted in 6 patients during increased heart rate (HR; 3 with exercise and 3 with isoprenaline). ECGI maps at baseline and during the faster HR were compared. Simulation The O Hara-Rudy model of a human ventricular myocyte 17 was used in the simulations. Fast/late I Na and I to were replaced by formulations used previously to represent the prominent phase 1 repolarization notch characteristic of RVOT APs. 18 The model was paced to steady state at normal and slow rates (1000 beats at 1 and 0.5 Hz, respectively) with various combinations of reduced I Na and enhanced I to conductances (G Na and G to ). Combinations were selected to span a virtual Brugada severity space. APD was measured at 90% repolarization (APD90 in milliseconds). Statistical Analysis For every subject, the mean values for EGM variables within each epicardial segment were computed. All statistical tests were performed at the level of epicardial segments. Differences in variables among epicardial segments were compared by 1-way repeated-measures analysis of variance. When the assumption of sphericity was violated, Greenhouse-Geisser correction was performed. Using the Bonferroni method, pairwise comparisons between RVOT and other regions were conducted (5 tests in total). Continuous variables at baseline and increased HR were compared by paired t test. Continuous variables between BrS and control and between BrS and RBBB were compared by unpaired t test. The Satterthwaite modified t test was used for variables with unequal variances. All tests with P<0.05 were considered statistically significant. Statistical analysis was performed by using SPSS v19.

3 1952 Circulation June 2, 2015 Results BrS Abnormal EGM Characteristics and Localization Table 1 summarizes the values of EGM parameters in each epicardial segment for baseline HR. Pairwise multiple comparisons between the RVOT and other regions suggest that the abnormal substrate is localized in the RVOT. Figure 1 shows EGM characteristics and localization for representative examples in 3 BrS patients (normal maps are provided for reference). STE (examples in Figure 1A) was observed in RVOT of all 25 patients (2.21±0.67 versus 0 mv in control, P<0.005), but rarely detected outside the RVOT. Three patients had lowmagnitude STE in the RV free wall and 2 had low-magnitude STE in the in the left ventricular (LV) free wall (range, mv). ECG leads with STE for all patients are identified in Table I in the online-only Data Supplement. Fifty-nine percent of EGMs in RVOT had STE>1 mv. EGM magnitude (Figure 1B) was lower in the RVOT (0.47±0.16 versus 3.74±1.60 mv in control, P<0.005) than in other regions (>2.5 mv). Fortysix percent of EGMs in the RVOT had voltage <2 mv. Four patients had low voltage in the RV free wall and 3 had low voltage in the LV base. Fractionated EGMs (Figure 1C) were present in the RVOT (number of deflections=2.97±0.69 versus 0 in control, P<0.005). Twenty-seven percent of EGMs in the RVOT had >2 deflections. Two patients had fractionated EGMs in the RV free wall. Figure 1D shows EGMs from locations marked by white dots in Figure 1C, representative of abnormal morphology of RVOT EGMs. Red arrows indicate low voltage and fractionated EGMs. BrS Activation Normal human epicardial activation patterns during SR were reported previously. 12,19 In general, earliest epicardial activation occurs in anterior RV (although there are many variants of location) 12,19 and the lateral LV base is most commonly the latest region to activate. Activation isochrone maps in Figure 2A show examples of how SR epicardial activation patterns were altered by the presence of BrS EP substrate. Patient BrS#4 had the earliest epicardial breakthrough (asterisk) over the RV free wall, from which activation propagated slowly (crowded isochrones) across the RVOT-RV free wall border. RV free wall and LV activated first, leaving the anterior RVOT to activate last (light blue). Patient BrS#15 had a similar activation pattern, with a broader wavefront and faster activation of the RV free wall. Slow conduction occurred in the RVOT (crowded isochrones in the blue region). Patient BrS#20 had the earliest activation in the anterior RV. Latest activation (dark blue) occurred at the lateral RVOT, nearly 65 ms after the anterior RV breakthrough. Of the 25 BrS patients imaged during SR, 20 demonstrated altered or delayed epicardial activation at the RVOT. Table 2 summarizes AT (mean±standard deviation) in each segment. AT in RVOT (82±18 versus 37±11 ms in control, P<0.005) was almost as late as that of LV base (80±16 ms), and was much delayed in comparison with the other 4 segments. In specific RVOT locations, AT was as late as 150 ms. AD (the time needed for activation of a defined region) was calculated for the RVOT (36±16 ms), the RV free wall (16±3 ms), the entire RV (40±14 ms), the entire LV (27±5 ms), and both ventricles (51±10 ms). RVOT activation took much longer than RV free wall or LV. AD of the RVOT accounted for 71% of total AD in both ventricles, highlighting the slow activation of the RVOT. BrS Repolarization Figure 2B and 2C shows representative ARI and RT maps from 3 BrS patients. Table 2 provides ARI and RT in each epicardial segment. ARI prolongation (318±32 versus 241±27 ms in control, P<0.005) and RT prolongation (381±30 versus 311±34 ms in control, P<0.005) were observed primarily in the RVOT, but also extended into adjacent neighboring regions of the RV free wall and the LV free wall. Steep epicardial ARI gradients and RT gradients (red arrows) occurred mostly at the RVOT-RV free wall or RVOT-LV free wall borders. These gradients also existed within the RVOT. Transitions of colors from blue to red across these regions reflect large differences in ARI and RT of 70 to 140 ms, leading to very steep localized ARI gradients (105±24 versus 7±5 ms/cm in control, 12 P<0.005) and RT gradients (96±28 versus 7±6 ms/cm in control, P<0.005). In patient BrS#15, the gradients between the anterior and lateral RVOT ARI map are not apparent in the RT map. This is because the anterior RVOT activated about 40 ms later than the lateral RVOT. This long delay masks the ARI differences between these 2 regions. However, in regions that activate without conduction delay (eg, RV free wall) RTs and their dispersion are primarily determined by ARI and ARI dispersion, independent of conduction. The patterns in RT maps closely follow the ARI patterns in such cases. Effects of Increased HR in BrS ECGI was performed in 6 BrS patients during increased HR (mean increase from 71 to 133 bpm). Figure 3 shows the Table 1. ECGI-Derived Parameters for BrS Patients: EGM Properties RVOT RV Free Wall RV Apex LV Base LV Free Wall LV Apex Mean peak STE, mv 2.21± ±0.09* 0* 0* 0.08±0.04* 0* Mean EGM magnitude, mv 0.47± ±0.94* 3.77±0.85* 2.99±1.17* 3.14±0.81* 5.88±1.18* (fractionated) Mean EGM fractionation, No. of deflections 2.97± ±0.80* 0* 0* 0* 0* Variables presented as mean±sd. BrS indicates Brugada syndrome; ECGI, ECG imaging; EGM, electrogram; LV, left ventricular; RV, right ventricular; RVOT, right ventricular outflow tract; SD, standard deviation; and STE, ST-segment elevation. *P<0.005 when comparing other segments with RVOT.

4 Zhang et al Cardiac EP Substrate in Brugada Syndrome Patients 1953 Figure 1. Abnormal epicardial electrograms (EGMs) characteristics and localization. A, Peak ST-segment elevation (STE) magnitude map. Insets show ECG lead V2. B, EGM magnitude map (EMM). C, EGM deflection map (EDM) showing number (#) of low-amplitude deflections. BrS maps are in 3 right columns; the left column shows corresponding maps from a healthy subject for reference. D, Unipolar EGMs from locations marked by white dots in C. 1, Anterior RVOT; 2, lateral RVOT; 3, RV free wall; 4, RV apex; 5, LV free wall (EGMs from other LV sites are also normal). Red traces show the time derivatives of fractionated QRS. The derivatives approximate bipolar EGMs and emphasize fractionation. Maps are shown in anterior view. Each BrS column shows maps/egms from 1 patient identified by BrS#. Red arrows point to low voltage and fractionated EGMs. BrS indicates Brugada syndrome; LA, left atrium; LV, left ventricle; PT, pulmonary trunk; RA, right atrium; RV, right ventricle; and RVOT, right ventricular outflow tract. effects of increased HR (from 72 to 115 bpm) in 1 representative example. Although the activation sequence remained largely unchanged (Figure 3A), RVOT activation was further delayed with HR increase (AD, from 32±7 ms to 38±10 ms), and RVOT EGMs had more fractionation and lower voltage (Figure 3D through 3F). This change of RVOT EGMs was observed in 4 of 6 patients (no significant change in the other 2 patients). When HR increased, ARIs (Figure 3B, corrected for HR) were shorter over the entire heart, but the RVOT ARIs decreased more than in other regions (by 50 ms in comparison with 25 ms in RV free wall). The regions with steep ARI gradients persisted, but the magnitude of the ARI gradient decreased (from 117 to 96 ms/cm). Reductions in RT and RT gradients were also observed during increased HR. STE (Figure 3C) in RVOT was significantly reduced (from 2.0 to 0.5 mv). Similar changes in ARI, RT, and STE were observed in all 6 patients (Table 3). Comparison Between BrS and Non-BrS RBBB ECGI was conducted during sinus rhythm in 6 non-brs patients with RBBB, without structural heart disease. Epicardial activation patterns, ARI maps, and EGM morphologies were analyzed and compared with those of the BrS patients. This comparison is of clinical importance, because the diagnosis of BrS in the presence of an RBBB ECG could be challenging. This is demonstrated in Figure 4A, where the Brugada ECG pattern for BrS#10 is masked by an RBBB pattern. Figure 4 shows representative data from 4 patients (2 BrS and 2 RBBB). BrS#4 had a spontaneous Brugada ECG pattern, whereas the ECG of BrS#10 showed an atypical RBBB pattern. Table IV in the online-only Data Supplement summarizes the differences of ECGI parameters between all BrS and RBBB patients. In RBBB patients, activation of the entire RV was delayed, with a long conduction delay of 35 ms across the interventricular septum (crowded isochrones; black arrow). In contrast, delayed activation was confined to the RVOT of BrS patients. Normal RV epicardial breakthrough (indicative of normal conduction system participation) was observed in all BrS patients, but not in any RBBB patient. BrS patients had a much longer AD in the RVOT than RBBB patients (36±16 ms BrS versus 14±5 ms RBBB; P<0.005), accounting for most of the AD in the entire RV (40±14 ms BrS versus 24±6 ms RBBB; P<0.005). Unlike BrS, RBBB T-wave inversion in the RVOT and RV was not accompanied by ARI prolongation (RBBB=246±25 ms, BrS=326±30 ms, P<0.005), steep ARI dispersion (RBBB=8±4 ms/cm, BrS=105±24 ms/ cm, P<0.005), or steep RT gradients (RBBB=6±3 ms/cm, BrS=96±28 ms/cm, P<0.005); ARI and RT maps were uniform. This demonstrates that T-wave inversion per se does not indicate the presence of repolarization gradients. The abnormal EGM characteristics specific for BrS (STE and lowamplitude fractionation) were not observed in RBBB patients. It is noteworthy that ECGI unmasked the presence of BrS in BrS#10, who had an RBBB pattern on the body surface ECG but BrS substrate in the RVOT, as revealed by ECGI. Discussion This is the first panoramic ventricular mapping study in BrS patients, made possible by ECGI. The panoramic mapping identified the RVOT as the region of abnormal EP substrate

5 1954 Circulation June 2, 2015 Figure 2. Activation and repolarization during sinus rhythm. A, Activation times isochrone maps (AT). Insets, zoom on the RVOT. B, Activation-recovery interval maps (ARI). C, Recovery time (RT) maps. Epicardial breakthroughs are indicated by asterisks. Isochrones are depicted in thin black lines. Black arrows in the RVOT zoom maps of A point to slow conduction indicated by crowded isochronal lines. Red arrows in B and C point to regions with steep repolarization gradients. BrS indicates Brugada syndrome; LA, left atrium; LV, left ventricle; PT, pulmonary trunk; RA, right atrium; RV, right ventricle; and RVOT, right ventricular outflow tract. with the following properties: (1) abnormal EGMs characterized by STE and inverted T wave, reduced amplitude and fractionation; (2) conduction delays and regions of slow conduction; (3) prolonged repolarization and steep repolarization gradients; (4) reduced STE, increased fractionation, decreased RVOT ARI and RT, decreased ARI and RT gradients with increased HR. These properties are significantly different than in healthy controls (Table 4) and in patients with RBBB. Abnormal EGM Characteristics and Localization The RVOT has been suggested as the origin of arrhythmic activity in BrS patients. 1,6,8,20,21 Our observation of substrate localization exclusively to the RVOT in BrS patients provides crucial evidence for the RVOT dominant role in clinical BrS. This is in contrast to a recent ECGI study of the long QT syndrome, where abnormal repolarization substrate was spread over the entire epicardium. 15 ECGI revealed RVOT EGMs with coved morphology similar to the typical Brugada ECG morphology in the body surface right precordial leads (Figure 1). Additionally, the EGMs were of low amplitude and fractionated. These EGM properties, reconstructed noninvasively by ECGI, were consistent with those obtained from invasive electroanatomical mapping. 6,20 The ECGI panoramic mapping determined that the majority of these Brugada EGMs were confined to the RVOT; only rarely were they detected close to the RVOT, in adjacent neighboring regions of the RV or LV free wall. Therefore, our study adds insight to the observation from invasive epicardial mapping and ablation, that the Brugada morphology normalized following ablation in the anterior RVOT. 6 This EGMs localization confirms that the RVOT is the origin of the Brugada-type ECG morphology in the body surface right precordial leads. Substrate for Abnormal Conduction BrS has been considered a primary electric disease, because structural abnormalities have not been detectable by noninvasive clinical imaging (eg, echocardiography or MRI). Recently, RV interstitial derangements (fibrosis, fatty infiltration) have been found in endomyocardial biopsies of BrS patients, 7 and in the explanted heart of a BrS patient carrying an SCN5A mutation, as well. 22 Fibrosis and fatty infiltration have been associated with decreased electric coupling, discontinuous propagation, and reduced conduction velocity. 23 In the normal heart, excitability and conduction are dominated by I Na. In BrS, where I Na is impaired, I to becomes a significant determinant of excitability, opposing the depolarizing effect of I Na. This is especially true in the RVOT, where I to is expressed with very high density. In this region, the balance between reduced depolarizing I Na and large repolarizing I to results in a reduced excitatory current, which, on the background of a structurally abnormal substrate, can support long localized conduction delays and discontinuous slow conduction, conditions that facilitate sustained reentrant arrhythmias. 23 Experimental, theoretical, and clinical studies have shown that low-magnitude, fractionated, and wide EGMs reflect slow, nonuniform, and discontinuous conduction through a structurally heterogeneous substrate associated with separation and reduced coupling of myocardial fibers. 24 Previous studies demonstrated of ability ECGI to reconstruct EGMs with these properties. 9,13,14 In this study, ECGI reconstructed Table 2. ECGI-Derived Parameters for BrS Patients: Activation and Repolarization Parameters RVOT RV Free Wall RV Apex LV Base LV Free Wall LV Apex Mean AT, ms 82±18 52±12* 57±7* 80±16 62±8* 58±10* Mean RT, ms 381±30 272±42* 263±30* 317±38* 279±31* 272±31* Mean ARI, ms 318±32 229±36* 221±37* 245±40* 222±31* 220±27* Variables presented as mean±sd. ARI indicates activation-recovery interval; AT, activation time; BrS, Brugada syndrome; ECGI, ECG imaging; LV, left ventricular; RT, recovery time; RV, right ventricular; RVOT, right ventricular outflow tract; and SD, standard deviation. *P<0.005 when comparing other segments with RVOT.

6 Zhang et al Cardiac EP Substrate in Brugada Syndrome Patients 1955 Figure 3. Effects of increased heart rate (HR). A, Activation isochrone maps (AT). B, Activationrecovery interval (ARI) maps. C, Peak ST-segment elevation (STE) magnitude maps. D, Electrogram deflection maps (EDMs), showing number (#) of deflection on EGM. E, Electrogram magnitude maps (EMM). F, EGMs from RVOT locations marked by white dots in D. Each panel shows the map at resting (75 bpm) and increased HR (115 bpm). BrS indicates Brugada syndrome; LA, left atrium; LV, left ventricle; PT, pulmonary trunk; RA, right atrium; RV, right ventricle; and RVOT, right ventricular outflow tract. low-magnitude, fractionated, and wide unipolar EGMs. The time derivatives (dv/dt) of unipolar EGMs approximate bipolar EGMs and emphasize fractionation (Figure 1D, red). The reconstructed unipolar EGMs and the approximated bipolar EGMs are consistent with those measured directly by catheters in BrS patients, 6,20 providing evidence for slow discontinuous conduction in the RVOT. The ECGI mapping of late RVOT activation, prolonged AD, the presence of regions of slow conduction, and altered wavefront propagation in comparison with normal (Figure 2A) provides additional evidence supporting the conduction hypothesis. Similar observations Table 3. ECGI-Derived Parameters for BrS Patients: Effects of Increased HR were made with invasive mapping. 6,20 Computer modeling of AP conduction in the presence of BrS mutant I Na also produced slow discontinuous conduction, causing spatial gradients of membrane potential during the AP plateau, and early repolarization phase and STE in the computed pseudo-ecg waveform. 25 Taken together, these data provide evidence for the presence of abnormal conduction in the RVOT of BrS patients. Antzelevitch and colleagues used a canine RV wedge preparation, where I to activator and sodium and calcium channel blockers were applied to pharmacologically simulate effects RVOT RV Free Wall RV Apex LV Base LV Free Wall LV Apex Mean AT, ms Baseline HR 84±14 57±16 61±8 84±11 65±10 60±7 Faster HR 81±19 56±15 63±7 84±10 63±8 58±8 Mean RT, ms Baseline HR 392±26 265±38 274±34 333±42 284±39 281±38 Faster HR 358±30* 251±35 265±29 320±36 275±33 274±37 Mean ARI, ms Baseline HR 328±35 222±41 224±42 256±44 229±37 231±24 Faster HR 287±33* 203±32 210±39 239±40 218±36 220±22 Mean peak STE, mv Baseline HR 2.56± ± ± Faster HR 0.84± ± ± Mean EGM fractionation Baseline HR 2.73± ± Faster HR 3.65± ± Variables presented as mean±sd. ARI indicates activation-recovery interval; AT, activation time; BrS, Brugada syndrome; ECGI, ECG imaging; EGM, electrogram; HR, heart rate; LV, left ventricular; RT, recovery time; RV, right ventricular; RVOT, right ventricular outflow tract; SD, standard deviation; and STE, ST-segment elevation. *P<0.05 in comparison with baseline HR. P<0.005 in comparison with baseline HR.

7 1956 Circulation June 2, 2015 Figure 4. Comparison between BrS and non-brs RBBB. A, 12-lead ECGs. B, Activation isochrone maps (AT). C, Activation-recovery interval maps (ARI). D, EGMs from the RVOT (top) and RV free wall (bottom). ECGs, maps, and EGMs are shown for 4 representative examples (BrS#4, spontaneous Brugada ECG pattern; BrS#10, BrS patient with RBBB ECG pattern; RBBB#2 and RBBB#3, non- BrS patients with RBBB ECG patterns). Epicardial breakthroughs are indicated by asterisks. Black arrows point to slow conduction in RVOT (BrS) or across the septum (RBBB). BrS indicates Brugada syndrome; EGM, electrogram; LA, left atrium; LV, left ventricle; PT, pulmonary trunk; RA, right atrium; RV, right ventricle; RBBB, right bundle-branch block; and RVOT, right ventricular outflow tract. of BrS, and to explain EGM fractionation and late potentials. 3,4 This preparation did not include structural abnormalities and conduction disturbances; it investigated the mechanism of EGM fractionation in the setting of abnormal repolarization alone. It demonstrated the ability to generate secondary deflections on the epicardial EGMs based on differences in the AP morphology at different epicardial locations during the AP repolarization phase. Following the phase 1 notch (which underlies the STE), APs can differ by the presence/amplitude of a dome and by the time of complete repolarization. The associated gradients can result in 2 late deflections on the EGM. 4 Because the deflections occur during late repolarization, the shortest coupling interval to the main EGM deflection (generated by the AP upstroke) was 200 ms. 4 In the patients studied here, and in patients studied by using catheters, 6 EGM fractionation shows different properties; it includes >2 deflections (many show 4 deflections) that are tightly coupled to the main deflection (<50 ms) and even occur before the main deflection. Moreover, the fractionation is amplified with increased heart rate. All of these properties point to fragmented slow conduction as a major contributor to EGM fractionation. Of course, abnormal repolarization can also contribute to fractionation of the EGM, 4 but the augmentation with increased heart rate identifies conduction as the major mechanism. Substrate for Abnormal Repolarization I to expression is nonuniform in the ventricular myocardium. Expression is highest in the epicardium and diminishes transmurally toward the endocardium, creating a phase 1 notch in epicardial, but not endocardial AP. A particularly large I to density is found in the epicardial RVOT. Reduced I Na in BrS on the background of the large I to exaggerates the phase 1 notch, and the resulting spatial gradients of membrane voltage can give rise to STE in ECG waveforms. The deep and wide notch delays the AP plateau and prolongs APD, potentially causing reversal of membrane voltage gradients and T-wave inversion. 3,5 In extreme cases (not recorded in this study), the membrane is repolarized to voltages that are too low for sufficient I Ca-L activation during phase 1 of the AP and cells repolarize prematurely, losing the AP plateau. 3 This biphasic behavior (APD prolongation followed by APD shortening in conditions of extreme severity) is supported by the computer simulations of Figure 5. ECGI was validated to accurately map regions of altered repolarization and steep repolarization gradients. 10 The ECGI mapping in this study (Figure 2) demonstrated delayed repolarization and prolonged epicardial APDs (based on reconstructed ARIs) in the RVOT of BrS patients. Short ARIs, closely coupled extrasystoles, or ventricular tachycardia were not recorded in any patient. Interestingly, direct recordings from endocardium and epicardium revealed longer ARIs in the epicardium than in the endocardium of BrS patients, suggesting prolongation of epicardial APD; ARI shortening was not reported. 26 A steep repolarization gradient existed at the RVOT borders, which could be attributable to nonuniform spatial expression of the mutation. However, even uniform spatial distribution of mutant-channel current can cause spatial ARI dispersion. APD is determined by a delicate balance of transmembrane currents and this dependence is nonlinear. Therefore, spatially uniform mutant I Na expression on the background of nonuniform electrophysiological profile (heterogeneous I to ) can cause large spatial variation of the APD. It is also possible that reduced coupling and fiber separation in the BrS substrate limit electric loading and help maintain the steep spatial repolarization gradients. Examining the Coexistence of Conduction and Repolarization Substrates Using Increased HR The rate dependence of STE and EGM fractionation in BrS may be helpful in unmasking the existence of abnormal repolarization and abnormal conduction, respectively, in the EP substrate. Decreased STE at increased HR supports the repolarization hypothesis, because it suppresses the AP phase 1 notch and associated voltage gradients. In this study, STE in the

8 Zhang et al Cardiac EP Substrate in Brugada Syndrome Patients 1957 thus allowing for shorter reentry pathways and stabilizing reentry. Together, these 2 properties provide conditions for sustained reentrant excitation. Their coexistence in the substrate of BrS patients has been suggested by other recent studies 27,28 ; it is consistent with the high incidence of arrhythmic sudden cardiac death. Interestingly, most arrhythmic events occur during bradycardia. Reduced STE at increased HR reflects less steep repolarization gradients; it reduces arrhythmogenicity because the probability for unidirectional block is reduced. Figure 5. Computer simulation of BrS effects on a human RVOT AP. At pacing rate of 1 Hz and 0.5 Hz, APD90 (APD at 90% repolarization) was plotted vs I to conductance increase (G to from 1.0- to 2.5-fold). Colors indicate different amounts of I Na conductance reduction (G Na from 100% to 10%). APs and their duration are shown for G to /G Na pairings selected to illustrate the behaviors of interest: control (A and I); AP prolongation (B, C, E, F, J, and L); premature repolarization and AP shortening (D, G, H, and M); alternating AP prolongation and shortening (D, E, and F, G, H). A through M relate the summary data above with each of the APs in the traces below. The simulations demonstrate biphasic changes of APD (prolongation followed by shortening). Prolongation was greater when I Na was reduced. With I Na loss, there was a critical degree of I to enhancement beyond which APs fully repolarized prematurely at phase 1, causing loss of plateau and APD shortening. Just below this critical degree, APs could alternate between premature repolarization and prolongation. AP indicates action potential; APD, action potential duration; BrS, Brugada syndrome; and RVOT, right ventricular outflow tract. RVOT decreased in all 6 patients with increased HR. In contrast, increased EGM fractionation with increased HR reflects the presence of slow discontinuous conduction, consistent with the conduction hypothesis. Increased RVOT EGM fractionation was observed in 4 patients at faster HR. Taken together, the effects of increased HR on STE and EGM fractionation support coexistence of these 2 mechanisms in the BrS substrate. Repolarization abnormality is the major contributor to STE, whereas conduction abnormality is the major contributor to EGM fractionation. From the arrhythmogenic perspective, the presence of steep repolarization gradients (dispersion) introduces asymmetry of excitability and thus conditions for unidirectional block. 23 Slow conduction shortens the wavelength of the reentrant AP, Comparison With Non-BrS RBBB BrS and RBBB differ greatly in their arrhythmogenicity, with BrS being highly arrhythmogenic. Therefore, a noninvasive method for distinguishing between these abnormalities is highly desirable. Despite the similarities in the 12-lead ECG, ECGI differentiated BrS from non-brs RBBB in several aspects: (1) BrS EGMs with STE, low voltage, and fractionation were not found in RBBB. (2) Nondelayed RV epicardial breakthrough was observed in all BrS patients, indicating involvement of a functioning RBB. RV breakthrough was absent in all RBBB patients, reflecting the defective conduction system. (3) In BrS, slow discontinuous conduction and delayed activation were confined to the RVOT and RVOT-RV border. In contrast, RBBB caused late activation of the entire RV because of a long conduction delay across the septum; there were no regional conduction delays within the RVOT/RV. (4) ARI prolongation and steep ARI gradients were observed in RVOT of BrS patients but not in RBBB patients. These differences could provide the basis for an ECGI-based differential diagnosis in the clinical setting. In addition, the BrS ECG pattern may be masked by an RBBB ECG pattern in some patients, causing difficulties in distinguishing between BrS and RBBB. This has led to invasive procedures (eg, RV pacing in Chiale intervention 29 ) that help unmask the Brugada phenotype in patients with RBBB. In this study, we demonstrated that ECGI can image the BrS substrate in such patients, providing a noninvasive method for the clinical diagnosis of BrS even when it is masked by an RBBB pattern on the body surface ECG. Conclusions Noninvasive ECGI reveals that the abnormal EP substrate in BrS patients is localized in the RVOT. Both abnormal repolarization and abnormal conduction are present in the substrate, leading to steep repolarization gradients and delayed activation. In addition, ECGI could differentiate BrS from RBBB based on differences in the patterns of activation and repolarization, and electrogram morphology. Table 4. ECGI-Derived Parameters for BrS Patients: Normal Values in RVOT Normal values in RVOT Peak STE=0 mv* EGM magnitude =3.74±1.60 mv* EGM fractionation=0* AT=37±11 ms* RT=311±34 ms* ARI=241±27 ms* Variables presented as mean±sd. ARI, activation-recovery interval; AT, activation time; BrS, Brugada syndrome; ECGI, ECG imaging; EGM, electrogram; HR, heart rate; LV, left ventricular; RT, recovery time; RV, right ventricular; RVOT, right ventricular outflow tract; SD, standard deviation; and STE, ST-segment elevation. *P<0.005 when comparing variables in RVOT between normal control and BrS patients. RVOT

9 1958 Circulation June 2, 2015 Limitations Because this is the first ECGI study of BrS patients, it does not differentiate between subgroups (genotype, symptoms, family history, etc) within the BrS population. ECGI data for the healthy controls were previously obtained with the same ECGI methodology; the data were not obtained at the time of this study. This study is limited to characterizing the BrS substrate from data obtained during SR and relating its properties to the BrS ECG and EGM phenotype. Results demonstrate the existence of substrate with steep dispersion of repolarization and slow conduction, conditions that facilitate reentrant arrhythmias. However, arrhythmias were not recorded in any of the patients and the study does not provide direct information about the mechanism of ventricular tachycardia in BrS patients. Typically, BrS I Na is reduced to 50% relative to control. Figure 5 shows simulated behavior for a large range of I Na reduction, paired with a range of I to to account for its high density in the RVOT. Deepening and broadening of the phase 1 notch, resulting in APD prolongation, is obtained for the entire range of I Na with G to 1.5 times control. The accentuated notch and resulting transmembrane voltage gradients contribute to STE in BrS EGMs. However, loss of the dome and APD shortening at 50% I Na required G to 3.3 times control (at 1-Hz pacing). A previously characterized BrS gain-of-function mutation in I to, L450F, was associated with a similarly large degree ( 3-fold) of current increase. 30 It should be mentioned that the simulations were conducted in an isolated cell. In the multicellular substrate of BrS hearts, electric loading on the AP dome during localized conduction delays (attributable to structural derangement) can contribute to premature repolarization, loss of the dome, and short APD. In the 25 patients studied here, short ARIs were never observed. Acknowledgments We greatly appreciate the able help and advice of Eric Novak with the statistical analysis. We also thank Julia Meyer for her assistance with recruiting the RBBB patients. Sources of Funding This study was supported by National Institutes of Health (NIH) National Heart, Lung, and Blood Institute grants R01-HL and R01-HL (to Dr Rudy) and by Washington University Institute of Clinical and Translational Sciences grant UL1-TR from the National Center for Advancing Translational Sciences of the NIH. Dr Rudy is the Fred Saigh Distinguished Professor at Washington University. Disclosures Dr Sacher received consultant fees and speaker honoraria from Biosense Webster, St. Jude Medical, Sorin Group, Medtronic and Biotronik. Dr Strom is a paid employee and stockholder of CardioInsight Technologies. Dr Cuculich received research support from National Institutes of Health and March of Dimes. Dr Silva received consultant fees and speaker honoraria from Medtronic, AliveCor. Dr Faddis received research support from Stereotaxis. Dr Cooper received consultant fees and speaker honoraria from Boston Scientific, St. Jude Medical, Medtronic, and Biotronik. Dr Hocini received lecture fees from Medtronic and St. Jude Medical, served on the advisory board for Medtronic, and is a stockholder of CardioInsight Technologies. Dr Haïssaguerre is a stockholder of CardioInsight Technologies and received lecture fees from Biosense Webster and Medtronic. Dr Scheinman received consultant fees and speaker honoraria from St. Jude Medical, Medtronics, Boston Scientific, and Biotronik. Dr Rudy cochairs the scientific advisory board, and holds equity in and receives royalties from CardioInsight Technologies. CardioInsight Technologies does not support any research conducted in Dr Rudy s laboratory. The other authors report no conflicts. References 1. Antzelevitch C, Brugada P, Borggrefe M, Brugada J, Brugada R, Corrado D, Gussak I, LeMarec H, Nademanee K, Riera ARP, Shimizu W, Schulze- Bahr E, Tan HL, Wilde AA. Brugada syndrome: Report of the second consensus conference endorsed by the Heart Rhythm Society and the European Heart Rhythm Association. Circulation. 2005;111: Brugada P, Brugada J. Right bundle branch block, persistent ST segment elevation and sudden cardiac death: a distinct clinical and electrocardiographic syndrome. A multicenter report. J Am Coll Cardiol. 1992;20: Antzelevitch C. Cellular basis and mechanism underlying normal and abnormal myocardial repolarization and arrhythmogenesis. Ann Med. 2004;36(suppl 1): Szél T, Antzelevitch C. Abnormal repolarization as the basis for late potentials and fractionated electrograms recorded from epicardium in experimental models of Brugada syndrome. J Am Coll Cardiol. 2014;63: doi: /j.jacc Aiba T, Shimizu W, Hidaka I, Uemura K, Noda T, Zheng C, Kamiya A, Inagaki M, Sugimachi M, Sunagawa K. Cellular basis for trigger and maintenance of ventricular fibrillation in the Brugada syndrome model highresolution optical mapping study. J Am Coll Cardiol. 2006;47: Nademanee K, Veerakul G, Chandanamattha P, Chaothawee L, Ariyachaipanich A, Jirasirirojanakorn K, Likittanasombat K, Bhuripanyo K, Ngarmukos T. Prevention of ventricular fibrillation episodes in Brugada syndrome by catheter ablation over the anterior right ventricular outflow tract epicardium. Circulation. 2011;123: doi: / CIRCULATIONAHA Frustaci A, Priori SG, Pieroni M, Chimenti C, Napolitano C, Rivolta I, Sanna T, Bellocci F, Russo MA. Cardiac histological substrate in patients with clinical phenotype of Brugada syndrome. Circulation. 2005;112: doi: /CIRCULATIONAHA Tukkie R, Sogaard P, Vleugels J, de Groot IK, Wilde AA, Tan HL. Delay in right ventricular activation contributes to Brugada syndrome. Circulation. 2004;109: doi: /01.CIR D1. 9. Burnes JE, Taccardi B, MacLeod RS, Rudy Y. Noninvasive ECG imaging of electrophysiologically abnormal substrates in infarcted hearts: a model study. Circulation. 2000;101: Ghanem RN, Burnes JE, Waldo AL, Rudy Y. Imaging dispersion of myocardial repolarization, II: noninvasive reconstruction of epicardial measures. Circulation. 2001;104: Ramanathan C, Ghanem RN, Jia P, Ryu K, Rudy Y. Noninvasive electrocardiographic imaging for cardiac electrophysiology and arrhythmia. Nat Med. 2004;10: doi: /nm Ramanathan C, Jia P, Ghanem R, Ryu K, Rudy Y. Activation and repolarization of the normal human heart under complete physiological conditions. Proc Natl Acad Sci U S A. 2006;103: doi: /pnas Cuculich PS, Zhang J, Wang Y, Desouza KA, Vijayakumar R, Woodard PK, Rudy Y. The electrophysiological cardiac ventricular substrate in patients after myocardial infarction: noninvasive characterization with electrocardiographic imaging. J Am Coll Cardiol. 2011;58: doi: /j.jacc Rudy Y. Noninvasive electrocardiographic imaging of arrhythmogenic substrates in humans. Circ Res. 2013;112: doi: / CIRCRESAHA Vijayakumar R, Silva JN, Desouza KA, Abraham RL, Strom M, Sacher F, Van Hare GF, Haïssaguerre M, Roden DM, Rudy Y. Electrophysiologic substrate in congenital Long QT syndrome: noninvasive mapping with electrocardiographic imaging (ECGI). Circulation. 2014;130: doi: /CIRCULATIONAHA Coronel R, de Bakker JM, Wilms-Schopman FJ, Opthof T, Linnenbank AC, Belterman CN, Janse MJ. Monophasic action potentials and activation recovery intervals as measures of ventricular action potential duration: experimental evidence to resolve some controversies. Heart Rhythm. 2006;3: doi: /j.hrthm O Hara T, Virág L, Varró A, Rudy Y. Simulation of the undiseased human cardiac ventricular action potential: model formulation and experimental

10 Zhang et al Cardiac EP Substrate in Brugada Syndrome Patients 1959 validation. PLoS Comput Biol. 2011;7:e doi: /journal. pcbi Clancy CE, Rudy Y. Na(+) channel mutation that causes both Brugada and long-qt syndrome phenotypes: a simulation study of mechanism. Circulation. 2002;105: Durrer D, van Dam RT, Freud GE, Janse MJ, Meijler FL, Arzbaecher RC. Total excitation of the isolated human heart. Circulation. 1970;41: Postema PG, van Dessel PF, de Bakker JM, Dekker LR, Linnenbank AC, Hoogendijk MG, Coronel R, Tijssen JG, Wilde AA, Tan HL. Slow and discontinuous conduction conspire in Brugada syndrome: a right ventricular mapping and stimulation study. Circ Arrhythm Electrophysiol. 2008;1: doi: /CIRCEP Yokokawa M, Takaki H, Noda T, Satomi K, Suyama K, Kurita T, Kamakura S, Shimizu W. Spatial distribution of repolarization and depolarization abnormalities evaluated by body surface potential mapping in patients with Brugada syndrome. Pacing Clin Electrophysiol. 2006;29: doi: /j x. 22. Coronel R, Casini S, Koopmann TT, Wilms-Schopman FJ, Verkerk AO, de Groot JR, Bhuiyan Z, Bezzina CR, Veldkamp MW, Linnenbank AC, van der Wal AC, Tan HL, Brugada P, Wilde AA, de Bakker JM. Right ventricular fibrosis and conduction delay in a patient with clinical signs of Brugada syndrome: a combined electrophysiological, genetic, histopathologic, and computational study. Circulation. 2005;112: doi: / CIRCULATIONAHA Kléber AG, Rudy Y. Basic mechanisms of cardiac impulse propagation and associated arrhythmias. Physiol Rev. 2004;84: doi: / physrev Wit AL, Josephson ME. Fractionated electrograms and continuous electrical activity: fact or artifact. Card Electrophysiol Arrhyth. 1985: Bébarová M, O Hara T, Geelen JL, Jongbloed RJ, Timmermans C, Arens YH, Rodriguez LM, Rudy Y, Volders PG. Subepicardial phase 0 block and discontinuous transmural conduction underlie right precordial ST-segment elevation by a SCN5A loss-of-function mutation. Am J Physiol Heart Circ Physiol. 2008;295:H48 H58. doi: /ajpheart Nagase S, Kusano KF, Morita H, Nishii N, Banba K, Watanabe A, Hiramatsu S, Nakamura K, Sakuragi S, Ohe T. Longer repolarization in the epicardium at the right ventricular outflow tract causes type 1 electrocardiogram in patients with Brugada syndrome. J Am Coll Cardiol. 2008;51: doi: /j.jacc Tokioka K, Kusano KF, Morita H, Miura D, Nishii N, Nagase S, Nakamura K, Kohno K, Ito H, Ohe T. Electrocardiographic parameters and fatal arrhythmic events in patients with Brugada syndrome: combination of depolarization and repolarization abnormalities. J Am Coll Cardiol. 2014;63: doi: /j.jacc Lambiase PD, Ahmed AK, Ciaccio EJ, Brugada R, Lizotte E, Chaubey S, Ben-Simon R, Chow AW, Lowe MD, McKenna WJ. High-density substrate mapping in Brugada syndrome: combined role of conduction and repolarization heterogeneities in arrhythmogenesis. Circulation. 2009;120: doi: /CIRCULATIONAHA Chiale PA, Garro HA, Fernández PA, Elizari MV. High-degree right bundle branch block obscuring the diagnosis of Brugada electrocardiographic pattern. Heart Rhythm. 2012;9: doi: /j. hrthm Giudicessi JR, Ye D, Tester DJ, Crotti L, Mugione A, Nesterenko VV, Albertson RM, Antzelevitch C, Schwartz PJ, Ackerman MJ. Transient outward current (Ito) gain-of-function mutations in the KCND3-encoded Kv4.3 potassium channel and Brugada syndrome. Heart Rhythm. 2011;8: Clinical Perspective Brugada syndrome (BrS) is a hereditary cardiac disorder associated with an increased risk of fatal ventricular arrhythmias. BrS has been extensively studied at the molecular and cellular scales. However, the cardiac electrophysiological substrate that underlies the BrS ECG and supports ventricular arrhythmias in the intact heart of BrS patients has not been elucidated. This article reports the results of panoramic electrophysiological mapping using noninvasive ECG imaging in 25 BrS patients. The results indicate that the abnormal electrophysiological substrate is localized exclusively in the right ventricular outflow tract. The right ventricular outflow tract displays delayed activation, prolonged repolarization, and steep repolarization gradients. These findings reveal the existence of both abnormal repolarization and slow discontinuous conduction in the BrS substrate. Together, these properties provide conditions that promote sustained reentry. These observations have implications for the diagnosis and mechanism-based, targeted therapy of BrS arrhythmias. A related finding of clinical significance is the demonstrated capability of ECG imaging to differentiate BrS from right bundle-branch block based on differences in epicardial activation, repolarization, and electrogram morphologies.